There are numerous applications for fasteners which can withstand high torque from their associated tool. One such high torque transmission fastener/tool combination is known in connection with German Patent No. 1728574, commercially available under the trade name TORX.sup..RTM. or equivalent. The head of a TORX.sup..RTM. compatible fastener has a cavity shaped like a hollow cylinder. The inner surface of the cavity defines a continuous curve of uniformly distributed elevations and depressions. The elevations and depressions forming the inner surface of the cavity are parallel to an axis passing longitudinally through the center of the fastener and the cavity. Commercially available TORX.sup..RTM. compatible fasteners typically have six depressions and six elevations, forming a generally hexagonal shaped cavity. The corresponding TORX.sup..RTM. tool has an outer surface which defines a continuous curve where the elevations and depressions are parallel to the longitudinal axis of the tool. The uniformly distributed elevations and depressions of the tool are complementary to the inner surface of the fastener cavity.
The elevations and depressions increase the total surface area available for engagement between the fastener cavity and the driver, thereby increasing torque transmission capabilities. Traditional TORX.sup..RTM. tools, however, can only be inserted vertically into the fastener cavity along the center longitudinal axis of the fastener. Tools of this construction can not be pivoted or tilted with respect to the longitudinal axis of the fastener. This limitation is particularly problematic when the fastener is located in a difficult to reach location.
The prior art which has attempted to overcome this limitation has a number of shortcomings. U.S. Pat. No. 4,824,418, issued to Taubert discloses an articulated power transmission joint with a substantially spherical driver with alternating depressions and elevations on its surface which are complimentary to the inner surface of the cavity on a TORX.sup..RTM. compatible fastener. The structure disclosed by Taubert allows the axis of the driver to be pivoted with respect to the longitudinal axis of the fastener, while maintaining engagement of the respective elevations and depressions to rotate the fastener.
Due to its spherical structure, the drive element disclosed by Taubert has less surface area available for engagement with the elevations and depressions on the inner surface of the cavity than on traditional TORX.RTM. compatible tools, resulting in reduced torque transmission capabilities and potential damage to either part of the transmission device. When the drive element is pivoted in relation to the longitudinal axis of the fastener, the surface area available for engagement between the tool and the fastener cavity is further reduced.
In order to maximize torque transmitting capacity, the centerline dividing the driver into upper and lower halves should be below the centerline in the depth of the fastener cavity. The spherical nature of the Taubert driver is such that the centerline of the drive element tends to be substantially above the centerline in the depth of the fastener cavity. When the driver is pivoted at an angle with respect to the centerline of the fastener, the transmission of torque to the fastener causes the spherical driver to disengage or "walk out" of the fastener cavity.
In order to maximize the pivoting capabilities of the tool with respect to the center axis of the fastener cavity, the spherical driver has a narrower cross-section than would otherwise be necessary. The reduced diameter of the Taubert driver also results in a reduction in surface area of contact between the driver and the fastener, with a corresponding reduction in torque transmission capabilities.
Another consequence of the reduced diameter of the spherical driver is that the driver teeth contact the elevations in the fastener cavity close to the crests of the teeth, resulting in reduced torque transmitting capabilities and accelerated wear.
Finally, in order to accommodate pivoting of the driver with respect to the fastener cavity, the elevations or teeth must extend substantially along the entire surface of the driver. The higher the driver profile, the higher the teeth. However, to preserve the pivoting capabilities of a spherical driver, the increased height of the teeth requires a corresponding decrease in width. The decreased width of the teeth results in lower overall strength and reduced torque transmitting capabilities.
The present invention relates to an elliptical driver with an elliptical cross-section which provides increased surface area available for engagement between the drive element and the fastener, and greater torque transmission. The center point of the elliptical drive element remains below the centerline in the depth of the fastener cavity to prevent the drive element from disengaging with the fastener, even when used in a pivoted orientation.